IPC-D-279 EN.pdf - 第124页
terminations of copper/nickel/gold which are sheared after plating (e.g. exposed base metals on the edges of contacts). These conditions can also lead to tarnish creep, the extension of corrosion products of copper over …
plastic and gasket materials (up to 1% water by weight)
results in swelling; cyclic changes in humidity can result in
‘‘creeping’’ of the plastic or gasket material. Very low lev-
els of water vapor (low Relative Humidity or RH) allow
ElectroStatic Discharge (ESD) voltages to build up.
Chemisorption of water into polymer systems such as
molding compounds results in a lower T
g
, lower strength
and increased total thermal expansion.
L-5.0 FRETTING ‘‘CORROSION’’
This category of corrosion is a mishmash of failure of con-
tacts from two causes:
• In contrast to the good performance of gold-gold (gold
to gold) contacts at low contact pressures and low cir-
cuit energy levels or of tin-tin contacts at high contact
pressures and high circuit energy levels, gold-tin con-
tacts fail even at high pressures and high energy levels
due to the formation of gold/tin intermetallics with low
conductivity at the contact points due to small relative
motions between the contacts (micromotion). Sepa-
rable connectors and sockets for components such as
ICs have contributed to system failure when the gold/
tin intermetallics are formed. The micromotions can be
caused by power cycling of the component as well as
shock, vibration or temperature cycling of the system.
• In the presence of hydrocarbon vapors and relative
contact movement as in relays, platinum group metal
contacts catalyze the development of an insulating
polymeric film; the source of the vapors can be the
housing of a relay. In the presence of silicone vapors,
electrical arcing and relative contact movement, metal
contacts develop an insulating silica film.
L-6.0 CORROSION DESIGN CHECKLIST
• Identify process, service, rework and repair environ-
ments and chemicals, particularly:
• Rapid transitions between high temperature/high
humidity and low temperature with immediate or
continuing operation.
• Long term low or battery power operation at high
relative humidity.
• Mitigate effects of long term low power operation by
requiring that low power ICs and other components
with DC bias be free of delamination and cracking
between encapsulant and the surface of the IC or sub-
strate; risk sites are covered with void free laminate,
solder mask or conformal coating (CC). The CC
should demonstrate no delamination, mealing or vesi-
cation after exposure to high humidity/DC bias (e.g.
by absence of delamination with Scanning Acoustic
Microscopy).
• Assure that substrate conductor and component lead
spacings are suitable for the environmental conditions
of the product (humidity, contaminating dust, thermal
cycling, bias, corrosive gases) where no additional
conformal coating is required. Minimum spacings
from conductors to (conductor, mounting hole, compo-
nent terminals) and from barrels to inner plane con-
ductors and other barrels are used only where needed.
Also a DfM guideline.
• Identify and design against potential damage to insu-
lating or moisture barrier solder mask or conformal
coating by stresses such as high temperature, UV, and
ozone exposure; for partially cured coatings, very low
temperature can cause cracking.
• In environments with anticipated vibration or thermo-
mechanical movements due to temperature cycling,
separable connectors and contacts are immobilized.
Particularly applies to the active catalyst platinum
family, including gold, to prevent the formation of
insulating polymers from condensible carbonaceous
vapors. See also ‘‘fretting corrosion’’ below.
• Identify susceptible metals used in the components and
general design; for those metals, quantify performance
under anticipated corrosion stress.
• Design assemblies and choose components which are
easy to keep clean or are easy to clean. Minimum use
of EIAJ ‘‘O’’ clearance components, particularly
where using water soluble flux. Minimum use of dis-
crete component spacers which introduce multiple
crevices (sugar or adhesive spot under components is
OK). Also a DfM guideline.
• The assembly process requires maintenance of clean
condition of components and assemblies (e.g. by the
use of gloves during handling); clean ambient condi-
tions during laminate handling, lamination, storage;
cleaning of printed board prior to solder mask applica-
tion. There is no hydrophilic solvent (e.g. polyglycol)
in fluxes or soldering oil. Solvent or water mechani-
cally removed prior to air dry (e.g. air knives); non-
polar solvent used to remove organics which trap or
mask water soluble contaminants; polar solvents or hot
DI water used to remove ionizable contaminants; ultra-
sonic cleaning or high solvent velocity to get between
crevices. Where applicable, periodic SIR testing is
conducted to verify continued efficacy of the cleaning
method.
• Avoid all identified potential moisture traps associated
with components, including open cavity packages
sealed with polymers and low clearance packages over
bare conductors. Corrodible portions inside the cavity
are sealed with hydrophobic materials such as silicone.
Conductors are sealed with solder mask or conformal
coating prior to assembly or component clearance
increased to effect complete cleaning.
• Avoid all identified, exposed galvanic couples such as
IPC-D-279 July 1996
112
terminations of copper/nickel/gold which are sheared
after plating (e.g. exposed base metals on the edges of
contacts). These conditions can also lead to tarnish
creep, the extension of corrosion products of copper
over the gold. See a sample galvanic compatibility
table below.
• Identify mechanical stress levels in susceptible metal
parts (particularly formed terminations for possible
contribution to stress corrosion or plating discontinui-
ties); alternatively, metal parts are formed in the
annealed state and postplated.
• Identify susceptible ceramic package brazed and plated
terminations which may have brazed joints of metals
constituting galvanic couples, may have exposed
plated interfaces, and may have been or need to be
trimmed and/or formed after plating.
• Use components which do not release corrosive mate-
rials. Wet slug tantalum with sulfuric acid electrolyte is
not recommended for new designs. Avoid aluminum
electrolytics with dimethyl formamide electrolyte
which degrades solder mask and conformal coatings.
Orient vent plugs of unavoidable electrolytic capaci-
tors to minimize damage consequent to component
failure—face vent plugs away from substrate.
• Identify and if possible avoid galvanic corrosion
couples; these practices may mitigate consequences:
• Plating is to be complete with no exposed interfaces
in plating systems such as copper overplated with
solder, tin, or gold. Alternatively, interpose nickel
plating between the copper and the overplate.
• Interpose an intermediate compatible material. For
instance, for a steel screw fastened to aluminum,
interpose a washer plated with cadmium (an environ-
mental no-no) or use ‘‘active’’ stainless steel for the
screw where the S/S passivation tends to isolate the
screw.
• Selectively metallic plate as required for reliable
electrical contact between pressure contacts; e.g. gold
to gold or tin to tin but not gold to tin (to avoid
‘‘fretting’’ corrosion).
• Coat surfaces (with polymeric or conversion mate-
rial) for insulation and moisture exclusion.
• Anodically finish for insulation and moisture exclu-
sion but be very careful; the coating is thin and brittle
and entrapped residual anodic processing fluids are
corrosive.
• Design so that cathodic metal area is much smaller
than the anodic metal area.
• Assure that fretting ‘‘corrosion’’ has been minimized
or eliminated:
• The need for contact lubricants has been investigated
and satisfied.
• Contact finish and thickness/porosity/smoothness is
appropriate to the use environment, including fre-
quency of reconnections and current/voltage condi-
tions.
• Contacts are gold to gold or tin to tin but not gold to
tin.
• Card mounting stresses and flex circuit flexures
(static or dynamic vibration) are controlled by
clamps, screws, hold-downs; the stresses are not
transmitted to the connector or to the contacts.
• Identify conditions at possible electrochemical corro-
sion risk sites:
• bare metal
• tight spacing (very small diameter vias, complex
mechanical fitted parts, connectors, switches, vari-
able elements...)
• relative humidity 65% or condensed water films and
droplets
• ionizable contamination
• conductors with DC potential (particularly between
leads of fine pitch elements and where pin assign-
ment is optional, avoid large potential differences
between adjacent pins of connectors);
• Pressure contacts are sealed from condensing mois-
ture, high humidity, and corrosive gases; conformal
rubber seals under continuous high pressure appears to
be effective. Be cautious of compression set effect of
rubbers at low temperatures.
• Avoid exposed silver plating, silver pastes, and silver
adhesives; overplate silver conductor material with
nickel or conformally coat or locate the component so
that water will not condense and run onto the silver.
Includes MLCC, DIP, rotary and slide switch, variable
resistor and buzzer packages.
• Tent all vias and PTHs on both ends if using active
water soluble flux (paste or liquid); alternatively, open
vias and PTHs do not terminate under a component
with tight clearance. A third alternative is to fill the
vias and PTHs with solder, epoxy or modified solder
mask/conformal coating material. These techniques
minimize barrel corrosion due to flux entrapment and
avoid test fixture corrosion and loss of SIR due to
drips of liquid flux.
• Hydrolyzable materials completely removed from the
PWA prior to application of any solder mask; similar
cleaning prior to any conformal coating application.
This is critical where water soluble flux systems are
used; otherwise, mealing and vesication can result
under high RH conditions.
• Avoid conductive anodic filament growth (CAF).
Evaluate printed board suppliers for delamination of
solder mask between conductors and laminate voids
July 1996 IPC-D-279
113
between conductors or barrels pre- and post-assembly
process stresses. To break possible CAF paths, use a
minimum of 2 plies for all layers and smooth and seal
board edges.
• printed board supplier’s inspection area is controlled
for humidity, temperature, cleanliness. Volume resis-
tance is a strong function of temperature changes, sur-
face resistance (SIR) is a strong function of humidity
and humidity changes, and delamination/measling/
vesication are functions of laminate cleaning and
cleanliness.
L-6.1 Galvanic Corrosion See also Sections 4 and 9,
ASM Electronic Materials Handbook, Volume 1, Packag-
ing, 1989; Contamination Effects on Electronic Products by
Carl Tautscher, Marcel Dekker, Inc., 1991, ISBN 0-8247-
8423-5. For sources of Ecorr vs. SCE, see Galvanic and
Pitting Corrosion-Field and Lab Studies, ASTM STP 576,
1976
Caution
Aluminum - 1% Silicon - 0.5% Copper alloy used for inte-
grated circuit metallization contains Al
2
Cu (Q phase)
which has a large oxidation (galvanic) potential with
respect to aluminum; in the presence of moisture, rapid
oxidation of aluminum occurs in the vicinity of copper
precipitates - and pits grow in the aluminum.
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